162 SHUPELDT. 



and each one is nearly as wide at its sternal end, where it is joined by 

 a costal rib, as it is at its angle. Those forming the first pair are the 

 shortest in the series; the sixth, seventh, and eighth pairs are of about 

 : equal length and are, at the same time, the longest ones of all. 



Seven of the leading pairs of ribs are joined to the sternum through the 

 intervention of costal ribs ; in the eighth pair the costal ribs are very long 

 ■ and attenuated, and lack but very little of meeting the distal extremity of 

 the mesosternum. After this they rapidly shorten, the ninth, tenth, and 

 sometimes the eleventh articulating in sequence with each other's lower 

 borders; the twelfth and thirteenth are thus Joined by a ligamentous 

 .membrane only, and are' practically floating costal ribs. (Figure 17.) 

 The first pair of eostals are the shortest of all that unite the vertebral 

 ribs with the anterior end of the presternum, occupying facets, one upon 

 either side. Just behind where the clavicles articulate. Like all the others 

 they are curved, the concavity of the curvature lookiag forward. From 

 the first to the last, or seventh, pair of eostals this curvature continues, 

 although it is here principally, indeed, almost entirely confined to the 

 outer third of the bone, especially in the last two or three pairs. They 

 become progressively longer as we pass backward, and all seem- to be 

 composed of true bone, although a little elementary in . character. The 

 distal pairs of eostals are still more cartilaginous, but there is evidence 

 of osseous tissue in all of them. At their sternal ends they articulate 

 upon facets situated between the Joints of the presternum and meso- 

 sternum, the sixth and the seventh articulating at the distal end of the 

 posterior Joint of the mesosternum. None of the eostals ever articulates 

 :with the xiphistemum. 



All the thoracic ribs articulate with the dorsal vertebrse in the way 

 usual among mammals and they present the common characters of these 

 bones. They are somewhat narrower in some specimens (1) than in 

 others (3, 3), but wide or narrow, any single rib in mid-series varies but 

 little in its own width from angle to costal articulation. All present the 

 usual curvature, although here it involves almost the entire continuity of 

 the bone and is most pronounced dorsad. 



If we select a "true rib" of the eighth pair as an example we find that 

 its capitulum is well developed; the elliptical double facet is rather large 

 and placed longitudinally on the bone. The "neck" is but moderately 

 constricted; the "tubercle" is but feebly pronounced, and owing to the 

 short transverse process of the dorsal vertebra, is quite close to the 

 capitulum. The >fangle" is but very, faintly indicated, and is entirely 

 absent in the last three pairs of true ribs. As in the rest of the series, 

 the "body" of the twelfth rib is very flat with rather sharp anterior and 

 posterior borders. These last in Steere's specimen are far more rounded. 

 Posteriorly and at the same time dorsad, there is a faint groove running 

 down the border of this rib. It harbors in Hf e' the intercostal vessels and 



